The present invention relates to a new and improved foundry aggregate binder system and composition containing a mixture of polyols. More particularly, the invention relates to a two-component foundry aggregate binder mixture comprised of (1) a resinous blend including a phenolic resin-based alkoxy polyol and one or more oxylated amines, and (2) a polyisocyanate. The improved foundry aggregate binder system of the present invention possesses the significant advantage of being substantially self-curing at normal foundry temperatures.
The use of curable urethane-based resins as foundry aggregate binders is well known in the art. In the foundry art, cores and molds used in the making of metal castings are generally prepared from a foundry mixture which is composed of an aggregate material which is primarily of foundry sand and a three-component binder system of (1) a polymerizable or curable phenolic-based resin, (2) a polyisocyanate and (3) a catalyst. The inclusion of the three-component urethane binder system with the aggregate forms the foundry mixture which is deposited in molds or shaped to a desired form and brought to a cured state by the presence of the catalyst to produce a molded, self-supporting shape or structure.
To effect curing of the phenolic resins used in foundry binder resin systems, a cross-linking agent such as an isocyanate which reacts with the alkylol end groups of the phenolic resin is used to produce a highly branched infusible polymer. In order to provide a rapid cure at normal foundry temperature, a catalyst is used to increase the activity of the cross-linking process to produce this infusible polymer.
One of the problems in the art has been the lack of ability of urethane resin binder systems to impart rapidly supportive tensile strength, known as green strength or stripping strength, to the aggregate mixture by the curing of the system at normal foundry temperature without the dependency on the inclusion of a catalyst in the resin binder system.
Special types of phenol formaldehyde resins are prevalent in the patent literature for use in the formation of cores or molds for use in the production of metal casting for foundry-shaped articles in which catalysts are used to effect a more rapid cure so that the cure takes place at normal foundry temperatures.
Exemplary of such patents are U.S. Pat. Nos. 3,409,579, 3,429,848, 3,432,457, 3,485,797, 3,676,392, 3,702,316, 3,726,867, 3,933,727 and 4,079,031, all of which disclose various types of foundry sand binder composition formed from a phenol formaldehyde resin, polyisocyanate, and a curing catalyst which may be in a gaseous or liquid form, such as, for example, amines; and liquid amines such as a tertiary amine. To be effective, these patented binders require a special type of phenol formaldehyde containing a benzylic ether linkage.
U.S. Pat. No. 4,079,031 discloses a foundry sand binder system which has as its binder resin component a resole-type phenolic and a minor portion of a polyether polyol which enters the cross-linked network through a linking with the end of an isocyanate chain that has its other end linked to a methylene attached to the phenol portion of the resole-type phenolic resin.
Apart from the field of foundry resins, the use of a mixture of oxyalkylated phenolic-based resin and oxyalkylated amines that react readily with polyisocyanates to form flame-resistant rigid foams is disclosed in U.S. Pat. No. 3,470,118, incorporated by reference and relied upon.